In attempting to minimize or avoid any requirement for a person washing to handle a soap or detergent container with dirty or soapy hands, liquid soap dispensers of the pump type, operated either manually or automatically by hand detecting devices, are often provided.
One known automatic liquid dispenser comprises a detecting means to detect one or both hands held out under the device and a pump driving means for driving the pump. Liquid such as soap is automatically expelled over one or both hands held out thereunder by automatically driving the pump driving means when a hand is detected by the detecting means, thereby avoiding any need to handle and possibly contaminate the device.
The conventional type of pump comprises a cylinder, a piston, and a check valve, and expels a liquid from the cylinder by reciprocating the piston manually or by driving means to open or close the valve in synchronism with the reciprocal movement of the piston. The liquid in the cylinder is expelled from the exhaust port by an forward or exhaust stroke of the piston and is introduced from a vessel (tank) into the cylinder by an return or intake stroke of the piston.
However, in such prior devices the entire amount of liquid soap is not expelled, as some liquid soap remains in a passageway extending from the cylinder to the exhaust port, dripping therefrom at a later time, contaminating, for example, a bottom surface of a dispenser, which is troublesome.